Light-sensitive polymer, method for preparing the same andmethod for forming patterns

ABSTRACT

A polymer having linear --Si--O--Si-- bonds and --Si--Si--Si-- bonds, or polysilane bonds more than trisilane bonds, sensitive to far ultraviolet rays. The light-sensitive polymer can be prepared by copolymerizing a dichlorodisiloxane and a dichlorosilane in an inert solvent in the presence of sodium. The polymer undergoes oxidation with oxygen plasma to form SiO 2  resistant to oxygen dry etching, exhibits absorption peaks only in far ultraviolet and is suitable for preparing a single layered resist or an upper resist of a two layered resist system.

This is a continuation of application Ser. No. 07/307,955 filed Feb. 9,1989, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a light-sensitive polymer sensitive tofar ultraviolet rays and suitable as a resist material for forming finepatterns of, for instance, semiconductor elements, a method forpreparing the same and a method for forming such a pattern using thelight-sensitive polymer.

Organopolysilanes, whose main chain is completely composed of siliconatoms, strongly absorb ultraviolet rays at 300 to 350 nm and thus showsensitivity to ultraviolet rays. Moreover, these compound undergooxidation with oxygen plasma to thus be converted to SiO₂ resistant tooxygen dry etching. Therefore, it has been attempted to adopt them asthe upper layer of a two layered resist system in ultravioletlithography techniques for manufacturing semiconductor elements or thelike and its effectiveness has been proved empirically.

On the other hand, the dramatic increase in density and degree ofintegration of today's semiconductor elements strongly requires thedevelopment of a method for forming a pattern having a width of not morethan 0.5 μm. To form such a fine pattern, it is believed that thelithography technique relying on the exposure to ultraviolet rays is nolonger ineffective and as a result, lithographic techniques utilizingelectron rays, X-rays or far ultraviolet rays have been investigated.However, the technique comprising exposure to electron rays makes theformation of fine patterns easy, but leads to low throughput; while withrespect to the technique comprising exposure to X-rays, there has notbeen developed any high-output source of X-rays and it is not expectedto further enhance the sensitivity of resist materials. On the contrary,in the technique comprising exposure to far ultraviolet rays, all theexisting equipment for a stepper can be employed by simply changing thelight source and optical systems, and high-output sources thereof suchas excimer lasers have already been developed. Therefore, the bestpossible lithography technique is one comprising the use of farultraviolet rays.

An important subject which has remained unsolved in the lithographytechnique using far ultraviolet rays is to develop a single layeredresist or an upper resist for two layered resist systems. As in theultraviolet lithography, the use of organopolysilanes has beeninvestigated, but the absorption at 300 to 350 nm which is important inthe ultraviolet exposure inhibits the bleaching effect and lowers thecontrast of the resultant pattern. Thus, there has not yet beendeveloped any such materials effective in the far ultraviolet exposure.

Under such circumstances, there has been a strong demand for thedevelopment of novel light-sensitive polymers which contain siliconatoms, strongly absorb only far ultraviolet rays and thus exhibitsensitivity to far ultraviolet rays.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a resistmaterial of a polymer which undergoes oxidation with oxygen plasma toform SiO₂ resistant to oxygen dry etching, which exhibits its absorptionpeaks only in far ultraviolet and which is suitable for preparing asingle layered resist or an upper resist of two layered resist systems.

Another object of the present invention is to provide a method forpreparing such a light-sensitive polymer.

The inventors of this invention have conducted various studies toachieve the aforementioned objects and have found that the objects canbe attained by providing a light-sensitive polymer comprising, in themolecule, linear --Si--O--Si-- bonds and --Si--Si--Si-- bonds, orpolysilane bonds more than trisilane. Thus, the inventors have completedthe present invention.

According to one aspect of the present invention, there is provided alight-sensitive polymer which comprises, in the molecule, at leastlinear ##STR1## or poly, more than tri-, silane bonds and which issensitive to far ultraviolet rays whose wave length is around 250 nm.

According to another aspect of the present invention, there is provideda method for preparing such a light-sensitive polymer which comprisesthe step of polymerizing dichlorodisiloxane represented by the followinggeneral formula (II): ##STR2## in an inert solvent in the presence ofsodium; or copolymerizing a dichlorodisiloxane represented by thegeneral formula (II) and dichlorosilanes represented by the followinggeneral formula (III): ##STR3## (wherein R₅ and R₆ are the same ordifferent and each represents a monovalent organic group) in an inertsolvent in the presence of sodium.

BRIEF EXPLANATION OF THE DRAWINGS

The present invention will hereinafter be explained in more detail withreference to the attached drawings, wherein;

FIGS. 1 and 4 are far ultraviolet absorption curves of an embodiment ofthe light-sensitive polymer of the present invention observed before andafter irradiating it with excimer laser rays; and

FIGS. 2 and 3 are schematic diagrams illustrating the steps of themethod for forming patterns according to one embodiment of thisinvention.

DETAILED EXPLANATION OF THE INVENTION

The light-sensitive polymer according to the present invention ischaracterized in that it comprises, in the molecule, at least linear##STR4## and examples thereof include those represented by the followinggeneral formula (I): ##STR5## In the formula (I), R₅ and R₆ are the sameor different and each represents a monovalent organic group) n being isa positive integer. More specifically, the monovalent organic group maybe a methyl, ethyl, propyl, isopropyl, n-butyl, n-hexyl, n-octyl,phenyl, tolyl or p-methoxyphenyl group etc.

The average molecular weight of the light-sensitive polymer of thisinvention desirably ranges from 1,000 to 1,000,000. This is because ifit is less than 1,000, it is impossible to form the polymer into a film,while if it is more than 1,000,000, the polymer cannot dissolve in asolvent. Therefore, the molecular weight of the light-sensitive polymershould be limited to the foregoing range in order to use the polymer asa resist used in the lithography techniques.

The light-sensitive polymer of the present invention can be prepared bycopolymerizing the aforesaid dichlorodisiloxane represented by thegeneral formula (II) and at least one aforementioned dichlorosilanerepresented by formula (III) in an inert solvent in the presence ofsodium.

In the foregoing method, the ratio of the compond (II) to the compound(III) used is preferably in the range of 1:1 to 4. Examples of the inertsolvents include toluene, xylene, mesitylene, n-octane and decalin. Thereaction is usually carried out at the atmospheric pressure in anitrogen gas atmosphere in the presence of 1 to 1.2 mol. of sodium permol. of the Si--Cl unit. The reaction temperature is preferably not lessthan the melting point of sodium, but the reaction also effectivelyproceeds at a temperature not more than the melting point of sodium ifthe reaction mixture is vigorously stirred.

In the above formula (III), examples of the groups R₅ to R₆ are the sameas those listed above in connection with the formula (I).

The light-sensitive polymer of the present invention can also beobtained by polymerizing the dichlorodisiloxane represented by formula(II) in an inert solvent in the presence of sodium. In thishomopolymerization, the same reaction conditions as those discussedabove can be employed.

The light-sensitive polymer of the present invention shows strongsensitivity only to far ultraviolet rays due to the presence of bonds##STR6## or poly, at least tri-, silane bonds and it undergoes oxidationwith oxygen plasma to form SiO₃, highly resistant to oxygen dry etchingas in the case of organopolysilanes. Therefore, the light-sensitivepolymer of the invention shows high resistance to the dry etching withoxygen plasma which is just required for such a resist pattern. Thus,the light-sensitive polymer can surely be applied as a resist materialin the far ultraviolet lithography technique. It has been experimentallyproven that the light-sensitive polymer provides ultrafine patterns ofhigh contrast and good shape in high sensitivity.

The present invention will hereunder be described more specifically withreference to the following non-limitative working Examples.

EXAMPLE 1

Methylphenyl dichlorosilane (765 g; 4.0 mol.) and the same amount ofdiethyl ether were introduced into a four-necked flask of 2 litersvolume equipped with a stirring machine, a reflux condenser, athermometer and a dropping funnel, followed by stirring the mixture,dropwise adding 36 g (2.0 mol.) of water through the dropping funnelover five hours under nitrogen atmosphere, and then refluxing themixture for an additional three hours. After refluxing the mixture,ether was distilled off and the starting material, methylphenyldichlorosilane (184 g; 0.96 mol.) was recovered by distilling thereaction mixture in vacuo to obtain 220 g (0.67 mol.) of1,3-dimethyl-1,3-diphenyl dichlorodisiloxane. Boiling Point=135°-138°C./2 mmHg.

EXAMPLE 2

A dispersion of 15.2 g (0.66 mol.) of sodium in 250 ml of xylene wascharged into a four-necked brown flask of 500 ml volume equipped with astirring machine, a reflux condenser, a thermometer and a droppingfunnel, followed by refluxing the dispersion, dropwise adding slowly, tothe flask, a mixed solution of 1,3-dimethyl-1,3-diphenyldichlorodisiloxane (49.1 g; 0.15 mol.) prepared in Example 1 anddiphenyldichlorosilane (38.0 g; 0.15 mol.) through the dropping funneland refluxing the mixture for 10 hours. After cooling the resultingmixture, the residual sodium was quenched with methanol, the mixture waswashed twice with a saturated aqueous solution of ammonium chloride andtwice with water and the xylene phase was separated and filtered. Xylenewas removed from the xylene phase by distillation and the resultantresidue was reprecipitated in tetrahydrofuran-isopropyl alcohol toobtain a polymer having the following repeating units as while powder(yield; about 25%). ##STR7##

Melting Point; 110°-120° C.

Weight Average Molecular Weight; 12,000

H-NMR spectra (CCl₄, δ ppm); -0.2-0.3 (m, Si--CH₃); 6.1-7.4 (m, ringprotons)

IR spectra (cm⁻¹); 3060, 3040, 3020, 2970, 1435, 1265, 1255, 1130, 1110,1100, 1030, 1000,

UV spectra (λ max.); 253 nm.

The ultraviolet absorption curves of the polymer observed before andafter irradiating it with KrF excimer laser rays of 248 nm are shown inFIG. 1. As seen from FIG. 1, the ultraviolet absorption decreases afterirradiating with light, which clearly shows the occurrence of aphotoreaction.

EXAMPLE 3

The polymer obtained in Example 2 was dissolved in toluene so that theconcentration of the resulting solution was 10% to form a resistsolution. In this respect, the solvent is not restricted to toluene sofar as it can dissolve the polymer.

The method for forming a resist pattern using the polymer of the presentinvention will hereunder be explained with reference to FIG. 2. Anunderlying resist 2 (for instance, RG 3900B available from HitachiChemical Co., Ltd.) was applied to the surface of a substrate 1 of, forinstance, a semiconductor with a rotary coater to form a resist film of2.0 micron thick (see FIG. 2 (a)). In this connection, substances to beetched such as insulating films and metal films are often applied to thesurface of the substrate 1. In addition, the materials for theunderlying resist are not limited to a specific one so far as they canbe etched by oxygen plasma. After heating the substrate to remove thesolvent (at 240° C. for 20 minutes), the resist solution prepared abovecontaining the light-sensitive polymer of the present invention wasapplied onto the underlying resist layer 2 with a rotary coater to forman upper resist layer 3 of 0.3 micron thick (see FIG. 2 (b)). Likewise,the substrate was heated (130° C. for 20 minutes) to remove the solventand then the upper resist layer 3 was selectively irradiated with pulsesof KrF excimer laser rays 4 (248 nm) through a mask carrying a desiredpattern (see FIG. 2(c)). Then, the exposed portions of the layer 3 wasdeveloped with ethanol to remove the same and to thus form a positiveworking upper resist pattern 3a (see FIG. 2(d)). In this connection, anydevelopers may be used so far as they are capable of dissolving out onlythe exposed portions. Finally, the underlying resist layer 2 wasselectively etched by oxygen plasma through the resist pattern 3aserving as a mask to transfer the pattern to the underlying resist layer2 (see FIG. 2 (e)) and thus the desired resist pattern 6 was formed(FIG. 2 (f)). At this stage, an oxide film (SiO₃) was formed on thesurface of the upper resist layer and thus the resistance to oxygen dryetching of the layer was enhanced. As a result, the resultant patternsshowed high contrast correctly corresponding to the mask design (0.4micron) without causing the reduction in the thickness of the resist.

EXAMPLE 4

In this Example, the light-sensitive polymer obtained in Example 2 wasused to form a single layered resist. The method for forming patternswill be explained below with reference to FIG. 3. 20% solution of thepolymer in toluene (resist solution) was applied onto the surface of asubstrate 1 of, for instance, a semiconductor with a rotary coater toform a resist film 3 of 1.0 micron thick (see FIG. 3 (a)). After heatingthe substrate (at 130° C. for 30 minutes) to remove the solvent, theresist layer 3 was irradiated with pulses of KrF excimer laser rays 4through a mask carrying a desired pattern (see FIG. 3 (b)). Finally, theresist layer 3 was developed with ethanol to remove the exposed portionsof the layer and to thus form a positive working pattern 6 (see FIG. 3(c)). The resultant pattern showed high contrast correctly correspondingto the mask design (0.4 micron), though the sensitivity thereof wasinferior to that of the two layered resist process.

EXAMPLE 5

The same procedures as in Example 2 were repeated except that a mixedsolution of 1,3-dimethyl-1,3-diphenyl dichlorodisiloxane (0.15 mol.),diphenyldichlorosilane (0.075 mol.) and methylphenyldichlorosilane(0.075 mol.) was used instead of the mixed solution of Example 2 toobtain a light-sensitive polymer having the following repeating units inan yield of about 20%. ##STR8##

The ultraviolet absorption curves of the light-sensitive polymerobserved before and after irradiating it with KrF excimer laser rays areshown in FIG. 4. FIG. 4 clearly indicates that a photoreaction surelyoccurred.

EXAMPLE 6

The same procedures as in Example 3 were repeated except for using thepolymer prepared in Example 5. As a result, patterns having highcontrast correctly corresponding to the mask design (0.4 micron) wereobtained in a sensitivity higher than that observed in Example 3.

EXAMPLE 7

The same procedures as in Example 4 were repeated except for using thepolymer prepared in Example 5. As a result, patterns having highcontrast correctly corresponding to the mask design (0.4 micron) wereobtained in a sensitivity higher than that observed in Example 4.

EXAMPLE 8

The same procedures as in Example 2 were repeated except that a weightof diphenyl dichlorosilane of a mixed solution was 76.0 g (0.30 mol.)instead of the mixed solution of Example 2 to obtain white powder havingthe following repeating units in an yield of about 18%. ##STR9##

Melting Point; 120°-130° C.

Weight Average Molecular Weight; 13,000

H-NMR spectra (CCl₄, δ ppm); -0.5-0.2 (m, Si--CH₃); 6.0-7.3 (m, ringprotons)

IR spectra (cm⁻¹); 3060, 3040, 3020, 2970, 1435, 1265, 1255, 1130, 1110,1100, 1030, 1000.

UV spectra (λ max.); 258 nm.

EXAMPLE 9

The same procedures as in Example 2 were repeated except that a mixedsolution of 49.1 g (0.15 mol.) of1,3-dimethyl-1,3-diphenydichlorosiloxane, and 29.6 g (0.15 mol.) ofcyclohexylmethyldichlorosilane was used instead of the mixed solution ofExample 2 to obtain white powder having the following repeating units inan yield of about 14%. ##STR10##

Weight Average Molecular Weight; 410,000

H-NMR spectra (CCl₄, δ ppm); -0.4-2.0 ##STR11## 6.6-7.5 (m, ringprotons)

IR spectra (cm⁻¹); 3060, 3040, 2920, 2850, 1450, 1435, 1255, 1110, 1005,1000

UV spectra (λ max.); 251 nm.

As discussed above in detail, the light-sensitive polymer of the presentinvention makes it possible to simultaneously achieve high sensitivityand high resolution if it is used as an upper resist layer in a twolayered resist process and to achieve high resolution by a simpleprocess if it is used in a single layered process. Moreover, the polymerof the present invention shows sensitivity and resolution higher thanthose of the conventional organopolysilanes and, therefore, it is clearthat the polymer has industrial value.

What is claimed is:
 1. A light-sensitive polymer for absorbing KrFexcimer laser rays, which comprises repeating units represented by thefollowing general formula [I]: ##STR12## wherein in the formula [I], Merepresents a methyl group, R₅ and R₆ are the same or different and eachrepresents a monovalent organic group selected from the group consistingof methyl, ethyl, propyl, isopropyl, n-butyl, n-hexyl, cyclohexyl,n-octyl, phenyl, tolyl and p-methoxyphenyl groups, n being a positiveinteger, said polymer being sensitive to about 250 nm wavelength KrFexcimer laser rays only and having a weight average molecular weight of1,000 to 1,000,000.
 2. The light-sensitive polymer of claim 1, whereineach of said organic group R₅ and said organic group R₆ is a phenylgroup, n being equal to
 1. 3. The light-sensitive polymer of claim 1,wherein each of said organic group R₅ and said organic group R₆ is aphenyl group, n being equal to
 2. 4. The light-sensitive polymer ofclaim 1, wherein said organic group R₅ is a methyl group and saidorganic group R₆ is a cyclohexyl group, n being equal to
 1. 5. Alight-sensitive polymer for absorbing KrF excimer laser rays, whichconsists of repeating units represented by the following general formula[IV]: ##STR13## wherein in the formula [IV], Me represents a methylgroup, each of p and q being a positive integer, said polymer beingsensitive to about 250 nm wavelength KrF excimer laser rays only andhaving a weight average molecular weight of 1,000 to 1,000,000.
 6. Amethod of preparing a light-sensitive polymer for absorbing KrF excimerlaser rays, having repeating units represented by the following generalformula [I]: ##STR14## wherein in the formula [I], Me represents amethyl group, R₅ and R₆ are the same or different and each of R₅ and R₆represents a monovalent organic group selected from the group consistingof methyl, ethyl, propyl, isopropyl, n-butyl, n-hexyl, cyclohexyl,n-octyl, phenyl, tolyl and p-methoxyphenyl groups, n being a positiveinteger, said polymer being sensitive to about 250 nm wavelength KrFexcimer laser rays only the method comprising the step of:copolymerizingdichlorodisiloxane represented by the following general formula [II]wherein Me is the same as the Me of the formula [I]: ##STR15## anddischlorosilane represented by the following general formula [III]:##STR16## wherein in the formula [III], each organic group R₅ andorganic group R₆ is the same as said organic group R₅ and said organicgroup R₆ of the formula [I], respectively, a molecular ratio of thecompound [II] to the compound [III] ranging from 1:1 to 1:4 with sodiumranging from 1 to 1.2 mol per mol of the unit Si--Cl in an inert solventselected from the group consisting of toluene, xylene, mesitylene,n-octane and decalin.
 7. The method of claim 6, wherein each of saidorganic group R₅ and said organic group R₆ of the formula [I] and theformula [III] is a phenyl group, n being equal to 1, the methodcomprising the step of copolymerizing1,3-dimethyl-1,3-diphenyldichlorodisiloxane and diphenyldichlorosilane,whose molecular ratio is 1:1, with the sodium in xylene.
 8. The methodof claim 6, wherein said organic group R₅ of the formula [I] and theformula [III] is a methyl group and said organic group R₆ of the formula[I] and the formula [III] is a phenyl group, n being equal to 1, themethod comprising the step of copolymerizing1,3-dimethyl-1,3-diphenyldichlorodisiloxane and diphenyldichlorosilane,whose molecular ratio is 1:2, with the sodium in xylene.
 9. The methodof claim 6, wherein said organic group R₅ of the formula [I] and theformula [III] is a methyl group and said organic group R₆ of the formula[I] and the formula [III] is a cyclohexyl group, n being equal to 1, themethod comprising the step of copolymerizing1,3-dimethyl-1,3-diphenyldichlorodisiloxane andcyclohexylmethyldichlorosilane, whose molecular ratio is 1:1, with thesodium in xylene.
 10. The method of claim 6, said polymer consisting ofrepeating units represented by the following general formula [IV]wherein Me represents a methyl group and each of p and g is a positiveinteger: ##STR17## the method comprising the step of copolymerizing1,3-dimethyl-1,3-diphenyldichlorodisiloxane, diphenyldichlorosilane andmethylphenyldichlorosilane, whose molecular ratios are 2:1:1,respectively, with the sodium in xylene.